1. Field
The present invention relates to technology for data storage.
2. Description of the Related Art
Non-volatile memory is widely used in various electronic devices such as cellular telephones, digital cameras, personal digital assistants, mobile computing devices, and non-mobile computing devices. Non-volatile memory allows information to be stored and retained even when an electronic device is not connected to a source of power (e.g., a battery). Three characteristics of a non-volatile memory device include its cost, energy consumption, and performance. The performance characteristic of a non-volatile memory device includes the time necessary to write information to the memory device and the time necessary to read information from the memory device.
Many commercially available non-volatile memory devices (e.g., NAND flash memory cards) contain two-dimensional arrays of non-volatile memory cells. The memory cells within a two-dimensional array form a single layer of memory cells and are selected via control lines in the X and Y directions. Two-dimensional arrays are typically formed on top of a silicon substrate. In contrast, the memory cells within a monolithic three-dimensional array form multiple “vertically aligned” layers of memory cells and are selected via control lines in the X, Y, and Z directions. A monolithic three-dimensional array is one in which multiple layers of memory cells are formed above a single substrate with no intervening substrates. Three-dimensional arrays of memory cells may be cheaper to fabricate than two-dimensional arrays of memory cells if the additional cost of forming multiple “vertically aligned” layers of memory cells is less than the cost of forming either wider or a greater number of two-dimensional arrays (i.e., if the cost of building memory layers vertically is less than the cost of building memory layers horizontally).
Three-dimensional memory arrays having more than one layer of memory cells have been formed by vertically aligning two-dimensional cross-point memory arrays. A cross-point memory array is one in which memory cells are placed at the intersection of a first set of control lines and a perpendicular second set of control lines. Exemplary three-dimensional memory arrays are described in U.S. Pat. No. 6,034,882 to Johnson, entitled “Vertically Stacked Field Programmable Nonvolatile Memory and Method of Fabrication,” and in U.S. Pat. No. 5,835,396 to Zhang, entitled “Three-Dimensional Read-Only Memory Array.”